Colonization and spreading dynamics of Lactiplantibacillus plantarum spoilage isolates on wet surfaces

The role of lactic acid bacteria, including Lactiplantibacillus plantarum, in food spoilage is well recognized, while the behavior of these non-motile bacteria on wet surfaces, such as those encountered in food processing environments has gained relatively little attention. Here, we observed a fast colony spreading of non-motile L. plantarum spoilage isolates on wet surfaces via passive sliding using solid BHI agar media as a model. We investigated the effect of physical properties of agar hydrogel substrate on the surface spreading of six L. plantarum food isolates FBR1-6 and a model strain WCFS1, using increasing concentrations of agar from 0.25 up to 1.5% (w/v). Our results revealed that L. plantarum strain FBR2 spreads significantly on low agar concentration plates compared to the other strains studied here (with a factor of 50-60 folds higher surface coverage), due to the formation of very soft biofilms with high water content that can float on the surface. The fast-spreading of FBR2 colonies is accompanied by an increased number of cells, elongated cell morphology, and a higher amount of extracellular components. Our finding highlights colonization dynamics and the spreading capacity of non-motile bacteria on surfaces that are relatively wet, thereby revealing an additional hitherto unnoticed parameter for non-motile bacteria that may contribute to contamination of foods by fast surface spreading of these bacteria in food processing environments.

Enhanced stress resistance of Bifidobacterium breve NRBB57 by induction of stress proteins at near-zero growth rates

Bifidobacterium breve is a common habitant of the human gut and is used as probiotic in functional foods. B. breve has to cope with multiple stress conditions encountered during processing and passage through the human gut, including high temperature, low pH and exposure to oxygen. Additionally, during industrial processing and in the gut, B. breve could encounter nutrient limitation resulting in reduced growth rates that can trigger adaptive stress responses. For this reason, it is important to develop culture methods that elicit resistance to multiple stresses (robustness) encountered by the bacteria. To investigate the impact of caloric restriction on robustness of the probiotic B. breve NRBB57, this strain was grown in lactose-limited chemostat cultures and in retentostat for 21 days, at growth rates ranging from 0.4 h^-1 to 0.00081 h^-1. Proteomes of cells harvested at different growth rates were correlated to acid, hydrogen peroxide and heat stress survival capacity. Comparative proteome analysis showed that retentostat-grown cells had significantly increased abundance of a variety of stress proteins involved in protein quality maintenance and DNA repair (DnaJ, Hsp90, FtsH, ClpB, ClpP1, ClpC, GroES, RuvB, RecA), as well as proteins involved in oxidative stress defence (peroxiredoxin, ferredoxin, thioredoxin peroxidase, glutaredoxin and thioredoxin reductase). Exposure to three different stress conditions, 45 °C, pH 3, and 10 mM H₂O₂, showed highest stress resistance of retentostat cells sampled at week 2 and week 3 grown at 0.0018 and 0.00081 h^-1. Our findings show that cultivation at near-zero growth rates induces higher abundance of stress defence proteins contributing to the robustness of B. breve NRBB57, thereby offering an approach that may support its production and functionality.

Role of substrate availability in the growth of Campylobacter co-cultured with extended spectrum beta-lactamase-producing Escherichia coli in Bolton broth

It is well-established that Extended-spectrum beta-lactamase-producing (ESBL-) Escherichia coli challenge reliable detection of campylobacters during enrichment in Bolton broth (BB) following ISO 10272-1:2017. The overgrowth of Campylobacter by ESBL-E. coli in the enrichment medium BB can lead to false-negative detection outcomes, but the cause for the growth suppression is yet unknown. A plausible reason could be the competition-induced lack of certain growth substrates. Therefore, this study aimed to investigate whether campylobacters and ESBL-E. coli compete for the same medium components and whether this is the cause for the observed growth repression. The availability of possible growth substrates in BB was determined and changes in their extracellular concentration were measured over time during mono-culture enrichment of C. jejuni, C. coli or ESBL-E. coli as well as in co-culture enrichments of campylobacters and ESBL-E. coli. Comparative analysis showed lactate and fumarate utilization by C. jejuni and C. coli exclusively, whereas ESBL-E. coli rapidly consumed asparagine, glutamine/arginine, lysine, threonine, tryptophan, pyruvate, glycerol, cellobiose, and glucose. Both campylobacters and ESBL-E. coli utilized aspartate, serine, formate, a-ketoglutarate and malate. Trends in compound utilization were similar for C. jejuni and C. coli and trends in compound utilization were rather comparable during enrichment of reference and freeze-stressed campylobacters. Since final cell densities of C. jejuni and C. coli in co-cultures were not enhanced by the addition of surplus l-serine and final cell densities were similar in fresh and spent medium, growth suppression seems not to be caused by a lack of substrates or production of inhibitory compounds. We hypothesized that oxygen availability was limiting growth in co-cultures. Higher oxygen availability increased the competitive fitness of C. jejuni 81-176 in co-culture with ESBL-E. coli in duplicate experiments, as cell concentrations in stationary phase were similar to those without competition. This could indicate the critical role of oxygen availability during the growth of Campylobacter and offers potential for further improvement of Campylobacter spp. enrichment efficacy.

Variability in lag-duration of Campylobacter spp. during enrichment after cold and oxidative stress and its impact on growth kinetics and reliable detection

Campylobacter jejuni and Campylobacter coli continue to be the leading cause of zoonotic gastroenteritis in the European Union, making reliable detection in food important. Low storage temperatures and atmospheric oxygen concentrations during food production can cause sub-lethal damage or transient non-culturability which is why ISO 10272-1:2017 includes an enrichment step to repair cell damage and increase cell concentrations, thereby supporting detection of campylobacters from foods. The aim of this study was to assess the variability in lag-duration of C. jejuni and C. coli during enrichment after different food-relevant stress treatments and evaluate its impact on growth kinetics and reliability of detection outcomes. Therefore, 13 C. jejuni and 10 C. coli strains were subjected to cold stress during refrigerated and frozen storage. Refrigerated storage did not significantly reduce culturability, but frozen storage reduced cell concentrations by 1.6 ± 0.1 log₁₀cfu/ml for both species. Subsequently, cells were enriched following ISO 10272-1:2017-A and cell concentrations were determined over time and lag-duration and growth rate were determined by fitting the Baranyi-model. Without prior stress treatment, mean lag-duration for C. jejuni and C. coli was 2.5 ± 0.2 h and 2.2 ± 0.3 h, respectively. Refrigerated storage increased lag-duration for C. jejuni to 4.6 ± 0.4 h and for C. coli to 5.0 ± 0.4 h and frozen storage increased lag-duration to 5.0 ± 0.3 h and 6.1 ± 0.4 h for C. jejuni and C. coli, respectively. Comparison of strain- and biological variability showed that differences in recovery after cold stress can be attributed mainly to strain variability since strain variability after refrigeration and freeze stress increased respectively 3-fold and 4-fold while biological variability remained constant. A subset of strains was also subjected to oxidative stress that reduced cell concentrations by 0.7 ± 0.2 log₁₀ cfu/ml and comparison of recovery patterns after oxidative and freeze stress indicated that recovery behaviour was also dependent on the stress applied. A scenario analysis was conducted to evaluate the impact of heterogeneity in outgrowth kinetics of single cells on the reliability of detection outcomes following ISO protocol 10272-1:2017. This revealed that a 'worst-case'-scenario for successful detection by a combination of the longest lag-duration of 7.6 h and lowest growth rate of 0.47 h^-1 still resulted in positive detection outcomes since the detection limit was reached within 32.5 h. This suggests that other factors such as competitive microbiota can act as a causative factor in false-negative outcomes of tested food samples.

Short communication: Growth of dairy isolates of Geobacillus thermoglucosidans in skim milk depends on lactose degradation products supplied by Anoxybacillus flavithermus as secondary species

Thermophilic bacilli such as Anoxybacillus and Geobacillus are important contaminants in dairy powder products. Remarkably, one of the common contaminants, Geobacillus thermoglucosidans, showed poor growth in skim milk, whereas significant growth of G. thermoglucosidans was observed in the presence of an Anoxybacillus flavithermus dairy isolate. In the present study, we investigated the underlying reason for this growth dependence of G. thermoglucosidans. Whole-genome sequences of 4 A. flavithermus strains and 4 G. thermoglucosidans strains were acquired, with special attention given to carbohydrate utilization clusters and proteolytic enzymes. Focusing on traits relevant for dairy environments, comparative genomic analysis revealed that all G. thermoglucosidans strains lacked the genes necessary for lactose transport and metabolism, showed poor growth in skim milk, and produced white colonies on X-gal plates, indicating the lack of β-galactosidase activity. The A. flavithermus isolates scored positive in these tests, consistent with the presence of a putative lactose utilization gene cluster. All tested isolates from both species showed proteolytic activity on milk plate count agar plates. Adding glucose or galactose to liquid skim milk supported growth of G. thermoglucosidans isolates, in line with the presence of the respective monosaccharide utilization gene clusters in the genomes. Analysis by HPLC of A. flavithermus TNO-09.006 culture filtrate indicated that the previously described growth dependence of G. thermoglucosidans in skim milk was based on the supply of glucose and galactose by A. flavithermus TNO-09.006.

Strain diversity and phage resistance in complex dairy starter cultures

The compositional stability of the complex Gouda cheese starter culture Ur is thought to be influenced by diversity in phage resistance of highly related strains that co-exist together with bacteriophages. To analyze the role of bacteriophages in maintaining culture diversity at the level of genetic lineages, simple blends of Lactococcus lactis strains were made and subsequently propagated for 152 generations in the absence and presence of selected bacteriophages. We first screened 102 single-colony isolates (strains) from the complex cheese starter for resistance to bacteriophages isolated from this starter. The collection of isolates represents all lactococcal genetic lineages present in the culture. Large differences were found in bacteriophage resistance among strains belonging to the same genetic lineage and among strains from different lineages. The blends of strains were designed such that 3 genetic lineages were represented by strains with different levels of phage resistance. The relative abundance of the lineages in blends with phages was not stable throughout propagation, leading to continuous changes in composition up to 152 generations. The individual resistance of strains to phage predation was confirmed as one of the factors influencing starter culture diversity. Furthermore, loss of proteolytic activity of initially proteolytic strains was found. Reconstituted blends with only 4 strains with a variable degree of phage resistance showed complex behavior during prolonged propagation.

Quantification of the impact of single and multiple mild stresses on outgrowth heterogeneity of Bacillus cereus spores

Outgrowth heterogeneity of bacterial spore populations complicates both prediction and efficient control of spore outgrowth. In this study, the impact of mild preservation stresses on outgrowth of Bacillus cereus ATCC 14579 spores was quantified during the first stages of outgrowth. Heterogeneity in outgrowth of heat-treated (90°C for 10 min) and non-heat-treated germinated single spores to the maximum micro-colony stage of 256 cells was assessed by direct imaging on Anopore strips, placed on BHI plates at pH7 and pH5.5, without and with added NaCl or sorbic acid (HSA). At pH7 non-heated and heat-treated germinated spores required 6h to reach the maximum microcolony stage with limited heterogeneity, and these parameters were only slightly affected with both types of spores when incubated at pH7 with added NaCl. Notably, the most pronounced effects were observed during outgrowth of spores at pH5.5 without and with added NaCl or HSA. Non-heat-treated germinated spores showed again efficient outgrowth with limited heterogeneity reaching the maximum microcolony stage after 6h at pH5.5, which increased to 12h and 16 h with added NaCl and HSA, respectively. In contrast, heat-treated spores displayed a strong delay between initial germination and swelling and further outgrowth at pH5.5, resulting in large heterogeneity and low numbers of fastest growers reaching the maximum microcolony stage after 10, 12 and 24h, without and with added NaCl or HSA, respectively. This work shows that Anopore technology provides quantitative information on the impact of combined preservation stresses on outgrowth of single spores, showing that outgrowth of germinated heat-treated spores is significantly affected at pH5.5 with a large fraction of spores arrested in the early outgrowth stage, and with outgrowing cells showing large heterogeneity with only a small fraction committed to relatively fast outgrowth.

Physiological and transcriptional response of Bacillus cereus treated with low-temperature nitrogen gas plasma

AIMS

This study was conducted to investigate the inactivation kinetics of Bacillus cereus vegetative cells upon exposure to low-temperature nitrogen gas plasma and to reveal the mode of inactivation by transcriptome profiling.

METHODS AND RESULTS

Exponentially growing B. cereus cells were filtered and put on agar plates. The plates, carrying the filters with the vegetative cells, were placed into low-temperature nitrogen gas plasma at atmospheric pressure. After different exposure times, the cells were harvested for RNA extraction and enumeration. The RNA was used to perform whole-transcriptome profiling using DNA microarrays. The transcriptome profile showed a large overlap with profiles obtained from conditions generating reactive oxygen species in B. cereus. However, excess radicals such as peroxynitrite, hydroxyl and superoxide could not be detected using radical-specific fluorescence staining. Lack of UV-specific responses including factors involved in DNA damage repair is in line with the absence of UV-specific emission in the afterglow of the nitrogen gas plasma as analysed using optical emission spectroscopy (OES).

CONCLUSIONS

Antibacterial activity of nitrogen gas plasma is not based on UV radiation. Exposure to nitrogen gas plasma leads to oxidative stress and inactivation of targeted cells. A secondary oxidative stress with the indicative formation of reactive oxygen species within cells could not be observed.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study represents the first investigation of differential gene expression on a genome-wide scale in B. cereus following nitrogen gas plasma exposure. This study may help to design economically feasible, safe and effective plasma decontamination devices.

Arginine metabolism in sugar deprived Lactococcus lactis enhances survival and cellular activity, while supporting flavour production

Flavour development in cheese is affected by the integrity of Lactococcus lactis cells. Disintegrated cells enhance for instance the enzymatic degradation of casein to free amino acids, while integer cells are needed to produce specific flavour compounds from amino acids. The impact of the cellular activity of these integer cells on flavour production remains to be elucidated. In this study we investigated whether lactose-deprived L. lactis cells that use arginine as an alternative energy source can extend cellular activity and produce more specific flavours. In cheese experiments we demonstrated that arginine metabolising cells survived about 3 times longer than non-arginine metabolising cells, which suggests prolonged cellular activity. Cellular activity and flavour production of L. lactis was further studied in vitro to enable controlled arginine supplementation. Comparable with the results found in cheese, the survival rates of in vitro incubated cells improved when arginine was metabolised. Furthermore, elongated cellular activity was reflected in 3-4-fold increased activity of flavour generating enzymes. The observed prolonged cellular activity resulted in about 2-fold higher concentrations of typical Gouda cheese flavours. These findings provide new leads for composing starter cultures that will produce specific flavour compounds.

Soya bean tempe extracts show antibacterial activity against Bacillus cereus cells and spores

AIMS

Tempe, a Rhizopus ssp.-fermented soya bean food product, was investigated for bacteriostatic and/or bactericidal effects against cells and spores of the food-borne pathogen Bacillus cereus.

METHODS AND RESULTS

Tempe extract showed a high antibacterial activity against B. cereus ATCC 14579 based on optical density and viable count measurements. This growth inhibition was manifested by a 4 log CFU ml(-1) reduction, within the first 15 min of exposure. Tempe extracts also rapidly inactivated B. cereus spores upon germination. Viability and membrane permeability assessments using fluorescence probes showed rapid inactivation and permeabilization of the cytoplasmic membrane confirming the bactericidal mode of action. Cooked beans and Rhizopus grown on different media did not show antibacterial activity, indicating the unique association of the antibacterial activity with tempe. Subsequent characterization of the antibacterial activity revealed that heat treatment and protease addition nullified the bactericidal effect, indicating the proteinaceous nature of the bioactive compound.

CONCLUSIONS

During fermentation of soya beans with Rhizopus, compounds are released with extensive antibacterial activity against B. cereus cells and spores.

SIGNIFICANCE AND IMPACT OF STUDY

The results show the potential of producing natural antibacterial compounds that could be used as ingredients in food preservation and pathogen control.

A Novel Method for Continuous Determination of the Intracellular pH in Bacteria with the Internally Conjugated Fluorescent Probe 5 (and 6-)-Carboxyfluorescein Succinimidyl Ester

A novel method based on the intracellular conjugation of the fluorescent probe 5 (and 6-)-carboxyfluorescein succinimidyl ester (cFSE) was developed to determine the intracellular pH of bacteria. cFSE can be taken up by bacteria in the form of its diacetate ester, 5 (and 6-)-carboxyfluorescein diacetate succinimidyl ester, which is subsequently hydrolyzed by esterases to cFSE in the cytoplasm. When Lactococcus lactis cells were permeabilized with ethanol, a significant proportion of cFSE was retained in the cells, which indicated that cFSE was bound intracellularly. Unbound probe could be conveniently extruded by a short incubation of the cells in the presence of a fermentable sugar, most likely by exploiting an active transport system. Such a transport system for cFSE was identified in L. lactis, Listeria innocua, and Bacillus subtilis. The intracellular pH in bacteria can be determined from the ratio of the fluorescence signal at the pH-sensitive wavelength (490 nm) and the fluorescence signal at the pH-insensitive wavelength (440 nm). This cFSE ratio method significantly reduced problems due to the efflux of fluorescent probe from the cells during the measurement. Moreover, the method described was successfully used to determine the intracellular pH in bacteria under stress conditions, such as elevated temperatures and the presence of detergents.

Characterization of germination and outgrowth of sorbic acid-stressed Bacillus cereus ATCC 14579 spores: phenotype and transcriptome analysis

Sorbic acid (SA) is widely used as a preservative, but the effect of SA on spore germination and outgrowth has gained limited attention up to now. Therefore, the effect of sorbic acid on germination of spores of Bacillus cereus strain ATCC 14579 was analyzed both at phenotype and transcriptome level. Spore germination and outgrowth were assessed at pH 5.5 without and with 0.75, 1.5 and 3.0 mM (final concentrations) undissociated sorbic acid (HSA). This resulted in distinct HSA concentration-dependent phenotypes, varying from reduced germination and outgrowth rates to complete blockage of germination at 3.0 mM HSA. The phenotypes reflecting different stages in the germination process could be confirmed using flow cytometry and could be recognized at transcriptome level by distinct expression profiles. In the absence and presence of 0.75 and 1.5 mM HSA, similar cellular ATP levels were found up to the initial stage of outgrowth, suggesting that HSA-induced inhibition of outgrowth is not caused by depletion of ATP. Transcriptome analysis revealed the presence of a limited number of transcripts in dormant spores, outgrowth related expression, and genes specifically associated with sorbic acid stress, including alterations in cell envelope and multidrug resistance. The potential role of these HSA-stress associated genes in spore outgrowth is discussed.

Transcriptional regulation of metabolic pathways, alternative respiration and enterotoxin genes in anaerobic growth of Bacillus cereus ATCC 14579

AIMS

To assess genes specifically activated during anaerobic growth that are involved in metabolism and pathogenesis of the foodborne pathogen Bacillus cereus.

METHODS AND RESULTS

Growth under anaerobic conditions in Brain Heart Infusion (BHI) broth revealed a reduced growth rate and lower yield as compared to growth under aerobic conditions. Subsequently, comparative transcriptome analysis showed specific genes induced under anaerobic conditions. These included novel genes identified for anaerobic growth of B. cereus, encoding metabolic pathways, such as the arginine deiminase pathway (ArcABDC), formate dehydrogenase (FdhF) and pyruvate formate lyase (Pfl), and alternative respiratory proteins, such as arsenate reductases. Notably, haemolytic enzyme encoding genes were induced during anaerobic growth, and enterotoxin genes were induced in high cell density transition and stationary phases of aerobic cultures.

CONCLUSIONS

These data point to induction of stress adaptation and pathogenicity factors and rearrangements of expression of metabolic pathways in response to oxygen limitations in B. cereus.

SIGNIFICANCE AND IMPACT OF THE STUDY

The reported changes in gene expression show that the foodborne pathogen B. cereus can adjust to anaerobic conditions, such as encountered in the human GI-tract.

Activity of ethanol-stressed Oenococcus oeni cells: a flow cytometric approach

AIMS

To study the effect of ethanol on Oenococcus oeni activity at the single cell level.

METHODS AND RESULTS

The active extrusion of the fluorescent probe carboxy fluorescein (cF) was used to assess the metabolic activity of ethanol-stressed O. oeni cells. Subsequent flow cytometric analysis revealed that O. oeni cells extrude the accumulated cF upon energizing with l-malic acid. However, O. oeni cells exposed to 12% (v/v) ethanol for 1 h showed a decreased capacity for active extrusion of cF. Moreover, two subpopulations could be distinguished, one of which being able to extrude cF and the other one remaining cF fluorescent. Growing cells in the presence of 8% (v/v) ethanol resulted in robust cells that maintained the capacity to actively extrude cF after being exposed to 12% (v/v) ethanol, which in turn correlated with the high levels of ATP observed in these ethanol stressed, malolactic fermentation (MLF) performing cells.

CONCLUSION

From our results, it becomes evident that active extrusion of cF can be used to assess malolactic activity in O. oeni.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present study provides information for the development of a rapid method to assess the malolactic activity of individual O. oeni cells performing MLF during wine production.

The growth limits of a large number of Listeria monocytogenes strains at combinations of stresses show serotype--and niche-specific traits

AIMS

The aim of this study was to associate the growth limits of Listeria monocytogenes during exposure to combined stresses with specific serotypes or origins of isolation, and identify potential genetic markers.

METHODS AND RESULTS

The growth of 138 strains was assessed at different temperatures using combinations of low pH, sodium lactate, and high salt concentrations in brain heart infusion broth. None of the strains was able to grow at pH < or = 4.4, a(w) < or = 0.92, or pH < or = 5.0 combined with a(w) < or = 0.94. In addition, none of the strains grew at pH < or = 5.2 and NaLac > or = 2%. At 30 degrees C, the serotype 4b strains showed the highest tolerance to low pH and high NaCl concentrations at both pH neutral (pH 7.4) and mild acidic conditions (pH 5.5). At 7 degrees C, the serotype 1/2b strains showed the highest tolerance to high NaCl concentrations at both pH 7.4 and 5.5. Serotype 1/2b meat isolates showed the highest tolerance to low pH in the presence of 2% sodium lactate at 7 degrees C. ORF2110 and gadD1T1 were identified as potential biomarkers for phenotypic differences.

CONCLUSIONS

Differences in growth limits were identified between specific L. monocytogenes strains and serotypes, which could in some cases be associated with specific genetic markers.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our data confirm the growth limits of L. monocytogenes as set out by the European Union for ready-to-eat foods and provides an additional criterion. The association of L. monocytogenes serotypes with certain stress responses might explain the abundance of certain serotypes in retail foods while others are common in clinical cases.

Germination of Bacillus cereus spores is induced by germinants from differentiated Caco-2 Cells, a human cell line mimicking the epithelial cells of the small intestine

Spores of 11 enterotoxigenic strains of Bacillus cereus isolated from foods and humans adhered with similar efficiencies to Caco-2 cells, whereas subsequent germination triggering was observed with only 8 of these strains. Notably, Hep-2 cells did not trigger germination, while spores of all strains displayed similar germination efficiencies in brain heart infusion broth.

Germination of Bacillus cereus spores adhered to stainless steel

Adhered spores of Bacillus cereus represent a significant part of the surface-derived contamination in processing equipment used in the dairy industry. As germinated spores lose their resistance capacities instantaneously, efficient germination prior to a cleaning in place treatment could aid to the disinfecting effect of such a treatment. Therefore, spores of B. cereus ATCC 14579 and that of the environmental isolate B. cereus CMCC 3328 were assessed for their germination behaviour when adhered to a stainless steel surface. A mixture of l-alanine and inosine initiated germination of adhered spores efficiently, resulting in 3.2 decimal logarithms of germination. Notably, implementation of a germination-inducing step prior to a representative cleaning in place procedure reduced the number of survivors with over 3 decimal log units, while an alkali treatment alone, as part of the cleaning in place procedure, did not show any effect on B. cereus spore viability. These results show that implementation of a germination step enhances the disinfection effect of currently used cleaning in place procedures.

Acid resistance variability among isolates of Salmonella enterica serovar Typhimurium DT104

AIMS

Acid resistance could be an indicator of virulence since acid resistant strains are able to better survive the human stomach passage and in macrophages. We studied the acid resistance of several Salmonella Typhimurium DT104 strains isolated from food and humans and identified cellular parameters contributing to the enhanced acid resistance of these isolates.

METHODS AND RESULTS

Acid resistance was tested in 37 Salmonella enterica Typhimurium serovar DT104 (S. Typhimurium DT104) strains. Acid adaptation at pH 5 followed by exposure for 2 h at pH 2.5 in the 27 human, nine nonhuman, and in two reference strains, revealed strong variation of acid survival. After 2 h at pH 2.5 six strains of S. Typhimurium DT104 were considered high acid resistant as they displayed a level of survival >10%, 14 strains were considered intermediate acid resistant (level of survival was <10% and >0.01%) and 19 strains were considered low acid resistant (level of survival <0.01%). Six strains were selected for further studies and proteomics revealed a relatively high amount of phase 2 flagellin in an acid-sensitive strain and a relatively high amount of the beta component of the H(+)/ATPase in an acid-resistant strain. Two strains were slightly more heat resistant possibly as the result of increased levels of DnaK or GroEL.

CONCLUSIONS

A significant difference could be detected between human and food isolates regarding their acid resistance; all high acid-resistant strains were human isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

S. Typhimurium DT104 is known for two decades and has a great impact on human health causing serious food-borne diseases. Our results suggest the existence of a positive correlation between acid resistance and pathogenicity in S. Typhimurium DT104 as all high acid-resistant strains were isolated from humans.

Selection of fluorescent probes for flow cytometric viability assessment of Listeria monocytogenes exposed to membrane-active and oxidizing disinfectants

The aim of this study was to select fluorescence methods for use as alternatives to plate counting to assess the viability of Listeria monocytogenes cells exposed to benzalkonium chloride (BAC) and hydrogen peroxide, two disinfectants with different mechanisms of action. A further aim of this study was to determine whether growth phase influences fluorescence labeling and whether it is possible to predict whether a probe will be a good viability indicator for cells exposed to a certain disinfectant on the basis of the mechanism of action of the disinfectant and the target of the fluorescent probe. The fluorescence methods used were labeling with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC; dehydrogenase activity), labeling with TOTO-1 iodide (TOTO; membrane-impermeant probe), and assessment of pH gradient maintenance in a low-pH buffer after labeling with the pH-sensitive probe 5-(and 6)-carboxyfluorescein succinimidyl ester (CFSE) (the pH(in) method). Growth phase influenced fluorescent labeling. However, the cutoff value for distinction between viable and nonviable cells was the same for both growth phases. The viability (determined by plate counts) of BAC-exposed cells correlated well with CTC labeling and TOTO exclusion. For both BAC-exposed and hydrogen peroxide-exposed cells, the pH(in) method gave a good qualitative indication of viability, sublethal damage, and cell death. CTC labeling and TOTO exclusion did not correlate with the viability of hydrogen peroxide-exposed cells. Our results demonstrate that even if the mechanism of action of a disinfectant is known, in some cases it is still difficult to predict whether a certain fluorescent probe is suitable for viability assessment. Thus, the proper selection of fluorescent probes for the assessment of the efficacy of antimicrobial agents is essential.

An antifungal compound produced by Bacillus subtilis YM 10-20 inhibits germination of Penicillium roqueforti conidiospores

AIMS

To identify and characterize an antifungal compound produced by Bacillus subtilis YM 10-20 which prevents spore germination of Penicillium roqueforti.

METHODS AND RESULTS

The antifungal compound was isolated by acid precipitation with HCl. This compound inhibited fungal germination and growth. Identification by HPLC and mass spectrometry analysis showed high similarity to iturin A. Permeabilization and morphological changes in P. roqueforti conidia in the presence of the inhibitor were revealed by fluorescence staining and SEM, respectively. CONCLUSOINS: The iturin-like compound produced by B. subtilis YM 10-20 permeabilizes fungal spores and blocks germination.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fluorescence staining in combination with flow cytometry and scanning electron microscopy are efficient tools for assessing the action of antifungal compounds against spores. Iturin-like compounds may permeabilize fungal spores and inhibit their germination.

The effect of the growth phase of Staphylococcus aureus on resistance to disinfectants in a suspension test

The influence of growth phase on the resistance of Staphylococcus aureus to the surface-active agents benzalkonium chloride and dodecylbenzyl sulfonic acid and the oxidizing agents sodium hypochlorite and hydrogen peroxide was studied. The resistances of cells in different growth phases were compared to those of solid medium cells grown according to the European phase I suspension test. Using cells from different growth phases (+/- 3 x 10(7) CFU ml(-1)), we found that decline-phase cells were the most resistant cells. However, the decline-phase cell suspension contained more than 90% dead cells. A 10-fold-diluted suspension with a total concentration of cells equal to that of the other cell suspensions still revealed decline-phase cells to be generally the most resistant cell type. However, the resistance was drastically reduced, indicating that the large proportion of dead cells provided significant protection to the viable decline-phase cells. Hydrogen peroxide resistance could be partly explained by the high catalase activity in the dead-cell fraction. Exponential-phase cells were less resistant than decline-phase cells, and, surprisingly, stationary-phase cells were the least resistant of the three. Cells grown according to the European phase 1 suspension test were never the most resistant cells. Their survival was 1 to 3 log units lower than that of the most resistant cells. These findings show that the solid-medium cells currently used in disinfectant tests are not the most resistant cells that can be used.

Cold shock proteins of Lactococcus lactis MG1363 are involved in cryoprotection and in the production of cold-induced proteins

Members of the group of 7-kDa cold-shock proteins (CSPs) are the proteins with the highest level of induction upon cold shock in the lactic acid bacterium Lactococcus lactis MG1363. By using double-crossover recombination, two L. lactis strains were generated in which genes encoding CSPs are disrupted: L. lactis NZ9000 Delta AB lacks the tandemly orientated cspA and cspB genes, and NZ9000 Delta ABE lacks cspA, cspB, and cspE. Both strains showed no differences in growth at normal and at low temperatures compared to that of the wild-type strain, L. lactis NZ9000. Two-dimensional gel electrophoresis showed that upon disruption of the cspAB genes, the production of remaining CspE at low temperature increased, and upon disruption of cspA, cspB, and cspE, the production of CspD at normal growth temperatures increased. Northern blot analysis showed that control is most likely at the transcriptional level. Furthermore, it was established by a proteomics approach that some (non-7-kDa) cold-induced proteins (CIPs) are not cold induced in the csp-lacking strains, among others the histon-like protein HslA and the signal transduction protein LlrC. This supports earlier observations (J. A. Wouters, M. Mailhes, F. M. Rombouts, W. M. De Vos, O. P. Kuipers, and T. Abee, Appl. Environ. Microbiol. 66:3756-3763, 2000). that the CSPs of L. lactis might be directly involved in the production of some CIPs upon low-temperature exposure. Remarkably, the adaptive response to freezing by prior exposure to 10 degrees C was significantly reduced in strain NZ9000 Delta ABE but not in strain NZ9000 Delta AB compared to results with wild-type strain NZ9000, indicating a notable involvement of CspE in cryoprotection.

DNA isolation protocols affect the detection limit of PCR approaches of bacteria in samples from the human gastrointestinal tract

A major concern in molecular ecological studies is the lysis efficiency of different bacteria in a complex ecosystem. We used a PCR-based 16S rDNA approach to determine the effect of two DNA isolation protocols (i.e. the bead beating and Triton-X100 method) on the detection limit of seven feces-associated bacterial species of different genera. Glycogen was used in these protocols to improve the precipitation of small concentrations of DNA in ethanol without affecting the sequential procedures. The PCR detection limit of 16S rDNA amplicons on agarose gel from the seven strains tested varied between 8.0 (+/- 1.3) x 10(4) and 4.3 (+/- 1.6) x 10(6) cells for the bead beating method, and between 8.0 (+/- 1.3) x 10(4) and 5.4 (+/- 0.7) x 10(8) cells for the Triton X-100 method. These large differences are most like due to the difference in cell lysis efficiency, since a competitive PCR experiment did not indicate any preference for gram negative, low G+C gram positive or high G+C gram positive bacteria. Denaturing gradient gel electrophoresis (DGGE) analysis was performed to investigate the effect of both DNA isolation protocols on the lysis efficiency of bacteria in fecal samples. A higher diversity in fecal samples was observed with the bead beating method than with the Triton-X100 method. Bands in the bead beating method-derived DGGE profiles corresponding to bands of cloned sequences of the Clostridium coccoides-Eubacterium rectale group and uncultured Fusobacterium prausnitzii were absent or had low intensity in the Triton X-100 method-derived profiles. The applicability of the bead beating method was further investigated by analyzing biopsy samples from the human colon which contain approximately 10(6) cells.

Fluorescent method for monitoring cheese starter permeabilization and lysis

A fluorescence method to monitor lysis of cheese starter bacteria using dual staining with the LIVE/DEAD BacLight bacterial viability kit is described. This kit combines membrane-permeant green fluorescent nucleic acid dye SYTO 9 and membrane-impermeant red fluorescent nucleic acid dye propidium iodide (PI), staining damaged membrane cells fluorescent red and intact cells fluorescent green. For evaluation of the fluorescence method, cells of Lactococcus lactis MG1363 were incubated under different conditions and subsequently labeled with SYTO 9 and PI and analyzed by flow cytometry and epifluorescence microscopy. Lysis was induced by treatment with cell wall-hydrolyzing enzyme mutanolysin. Cheese conditions were mimicked by incubating cells in a buffer with high protein, potassium, and magnesium, which stabilizes the cells. Under nonstabilizing conditions a high concentration of mutanolysin caused complete disruption of the cells. This resulted in a decrease in the total number of cells and release of cytoplasmic enzyme lactate dehydrogenase. In the stabilizing buffer, mutanolysin caused membrane damage as well but the cells disintegrated at a much lower rate. Stabilizing buffer supported permeabilized cells, as indicated by a high number of PI-labeled cells. In addition, permeable cells did not release intracellular aminopeptidase N, but increased enzyme activity was observed with the externally added and nonpermeable peptide substrate lysyl-p-nitroanilide. Finally, with these stains and confocal scanning laser microscopy the permeabilization of starter cells in cheese could be analyzed.

Analysis of the role of OpuC, an osmolyte transport system, in salt tolerance and virulence potential of Listeria monocytogenes

The success of Listeria monocytogenes as a food-borne pathogen owes much to its ability to survive a variety of stresses, both in the external environment prior to ingestion and subsequently within the animal host. Growth at high salt concentrations and low temperatures is attributed mainly to the accumulation of organic solutes such as glycine betaine and carnitine. We utilized a novel system for generating chromosomal mutations (based on a lactococcal pWVO1-derived Ori(+) RepA(-) vector, pORI19) to identify a listerial OpuC homologue. Mutating the operon in two strains of L. monocytogenes revealed significant strain variation in the observed activity of OpuC. Radiolabeled osmolyte uptake studies, together with growth experiments in defined media, linked OpuC to carnitine and glycine betaine uptake in Listeria. We also investigated the role of OpuC in contributing to the growth and survival of Listeria in an animal (murine) model of infection. Altering OpuC resulted in a significant reduction in the ability of Listeria to colonize the upper small intestine and cause subsequent systemic infection following peroral inoculation.

Flow cytometric assessment of viability of lactic acid bacteria

The viability of lactic acid bacteria is crucial for their applications as dairy starters and as probiotics. We investigated the usefulness of flow cytometry (FCM) for viability assessment of lactic acid bacteria. The esterase substrate carboxyfluorescein diacetate (cFDA) and the dye exclusion DNA binding probes propidium iodide (PI) and TOTO-1 were tested for live/dead discrimination using a Lactococcus, a Streptococcus, three Lactobacillus, two Leuconostoc, an Enterococcus, and a Pediococcus species. Plate count experiments were performed to validate the results of the FCM assays. The results showed that cFDA was an accurate stain for live cells; in exponential-phase cultures almost all cells were labeled, while 70 degrees C heat-killed cultures were left unstained. PI did not give clear live/dead discrimination for some of the species. TOTO-1, on the other hand, gave clear discrimination between live and dead cells. The combination of cFDA and TOTO-1 gave the best results. Well-separated subpopulations of live and dead cells could be detected with FCM. Cell sorting of the subpopulations and subsequent plating on agar medium provided direct evidence that cFDA labels the culturable subpopulation and that TOTO-1 labels the nonculturable subpopulation. Applied to cultures exposed to deconjugated bile salts or to acid, cFDA and TOTO-1 proved to be accurate indicators of culturability. Our experiments with lactic acid bacteria demonstrated that the combination of cFDA and TOTO-1 makes an excellent live/dead assay with versatile applications.

Effect of benzalkonium chloride on viability and energy metabolism in exponential- and stationary-growth-phase cells of Listeria monocytogenes

The difference in killing exponential- and stationary-phase cells of Listeria monocytogenes by benzalkonium chloride (BAC) was investigated by plate counting and linked to relevant bioenergetic parameters. At a low concentration of BAC (8 mg liter(-1)), a similar reduction in viable cell numbers was observed for stationary-phase cells and exponential-phase cells (an approximately 0.22-log unit reduction), although their membrane potential and pH gradient were dissipated. However, at higher concentrations of BAC, exponential-phase cells were more susceptible than stationary-phase cells. At 25 mg liter(-1), the difference in survival on plates was more than 3 log units. For both types of cells, killing, i.e., more than 1-log unit reduction in survival on plates, coincided with complete inhibition of acidification and respiration and total depletion of ATP pools. Killing efficiency was not influenced by the presence of glucose, brain heart infusion medium, or oxygen. Our results suggest that growth phase is one of the major factors that determine the susceptibility of L. monocytogenes to BAC.

Physiological function of exopolysaccharides produced by Lactococcus lactis

The physiological function of EPS produced by Lactococcus lactis was studied by comparing the tolerance of the non-EPS producing strain L. lactis ssp. cremoris MG1614 and an EPS producing isogenic variant of this strain to several anti-microbial factors. There was no difference in the sensitivity of the strains to increased temperatures, freezing or freeze-drying and the antibiotics, penicillin and vancomycin. A model system showed that EPS production did not affect the survival of L. lactis during passage through the gastrointestinal tract although the EPS itself was not degraded during this passage. The presence of cell associated EPS and EPS in suspension resulted in an increased tolerance to copper and nisin. Furthermore, cell associated EPS also protected the bacteria against bacteriophages and the cell wall degrading enzyme lysozyme. However, it has not been possible, so far, to increase EPS production using the presence of copper, nisin, lysozyme or bacteriophages as inducing factors.

Characterization of histatin 5 with respect to amphipathicity, hydrophobicity, and effects on cell and mitochondrial membrane integrity excludes a candidacidal mechanism of pore formation

Histatin 5 is a 24-residue peptide from human saliva with antifungal properties. We recently demonstrated that histatin 5 translocates across the yeast membrane and targets to the mitochondria, suggesting an unusual antifungal mechanism (Helmerhorst, E. J., Breeuwer, P., van't Hof, W., Walgreen-Weterings, E., Oomen, L. C. J. M., Veerman, E. C. I., Nieuw Amerongen, A. V., and Abee, T. (1999) J. Biol. Chem. 274, 7286-7291). The present study used specifically designed synthetic analogs of histatin 5 to elucidate the role of peptide amphipathicity, hydrophobicity, and the propensity to adopt alpha-helical structures in relation to membrane permeabilization and fungicidal activity. Studies included circular dichroism measurements, evaluation of the effects on the cytoplasmic transmembrane potential and on the respiration of isolated mitochondria, and analysis of the peptide hydrophobicity/amphipathicity relationship (Eisenberg, D. (1984) Annu. Rev. Biochem. 53, 595-623). The 14-residue synthetic peptides used were dh-5, comprising the functional domain of histatin 5, and dhvar1 and dhvar4, both designed to maximize amphipathic characteristics. The results obtained show that the amphipathic analogs exhibited a high fungicidal activity, a high propensity to form an alpha-helix, dissipated the cytoplasmic transmembrane potential, and uncoupled the respiration of isolated mitochondria, similar to the pore-forming peptide PGLa (Peptide with N-terminal Glycine and C-terminal Leucine-amide). In contrast, histatin 5 and dh-5 showed fewer or none of these features. The difference in these functional characteristics between histatin 5 and dh-5 on the one hand and dhvar1, dhvar4, and PGLa on the other hand correlated well with their predicted affinity for membranes based on hydrophobicity/amphipathicity analysis. These data indicate that the salivary protein histatin 5 exerts its antifungal function through a mechanism other than pore formation.

Physiological and regulatory effects of controlled overproduction of five cold shock proteins of Lactococcus lactis MG1363

The physiological and regulatory effects of overproduction of five cold shock proteins (CSPs) of Lactococcus lactis were studied. CspB, CspD, and CspE could be overproduced at high levels (up to 19% of the total protein), whereas for CspA and CspC limited overproduction (0.3 to 0.5% of the total protein) was obtained. Northern blot analysis revealed low abundance of the cspC transcript, indicating that the stability of cspC mRNA is low. The limited overproduction of CspA is likely to be caused by low stability of CspA since when there was an Arg-Pro mutation at position 58, the level of CspA production increased. Using two-dimensional gel electrophoresis, it was found that upon overproduction of the CSPs several proteins, including a number of cold-induced proteins of L. lactis, were induced. Strikingly, upon overproduction of CspC induction of CspB, putative CspF, and putative CspG was also observed. Overproduction of CspB and overproduction of CspE result in increased survival when L. lactis is frozen (maximum increases, 10- and 5-fold, respectively, after 4 freeze-thaw cycles). It is concluded that in L. lactis CSPs play a regulatory role in the cascade of events that are initiated by cold shock treatment and that they either have a direct protective effect during freezing (e.g., RNA stabilization) or induce other factors involved in the freeze-adaptive response or both.

Changes in glycolytic activity of Lactococcus lactis induced by low temperature

The effects of low-temperature stress on the glycolytic activity of the lactic acid bacterium Lactococcus lactis were studied. The maximal glycolytic activity measured at 30 degrees C increased approximately 2.5-fold following a shift from 30 to 10 degrees C for 4 h in a process that required protein synthesis. Analysis of cold adaptation of strains with genes involved in sugar metabolism disrupted showed that both the phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) subunit HPr and catabolite control protein A (CcpA) are involved in the increased acidification at low temperatures. In contrast, a strain with the PTS subunit enzyme I disrupted showed increased acidification similar to that in the wild-type strain. This indicates that the PTS is not involved in this response whereas the regulatory function of 46-seryl phosphorylated HPr [HPr(Ser-P)] probably is involved. Protein analysis showed that the production of both HPr and CcpA was induced severalfold (up to two- to threefold) upon exposure to low temperatures. The las operon, which is subject to catabolite activation by the CcpA-HPr(Ser-P) complex, was not induced upon cold shock, and no increased lactate dehydrogenase (LDH) activity was observed. Similarly, the rate-limiting enzyme of the glycolytic pathway under starvation conditions, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), was not induced upon cold shock. This indicates that a factor other than LDH or GAPDH is rate determining for the increased glycolytic activity upon exposure to low temperatures. Based on their cold induction and involvement in cold adaptation of glycolysis, it is proposed that the CcpA-HPr(Ser-P) control circuit regulates this factor(s) and hence couples catabolite repression and cold shock response in a functional and mechanistic way.

The role of cold-shock proteins in low-temperature adaptation of food-related bacteria

There is a considerable interest in the cold adaptation of food-related bacteria, including starter cultures for industrial food fermentations, food spoilage bacteria and food-borne pathogens. Mechanisms that permit low-temperature growth involve cellular modifications for maintaining membrane fluidity, the uptake or synthesis of compatible solutes, the maintenance of the structural integrity of macromolecules and macromolecule assemblies, such as ribosomes and other components that affect gene expression. A specific cold response that is shared by nearly all food-related bacteria is the induction of the synthesis so-called cold-shock proteins (CSPs), which are small (7 kDa) proteins that are involved in mRNA folding, protein synthesis and/or freeze protection. In addition, CSPs are able to bind RNA and it is believed that these proteins act as RNA chaperones, thereby reducing the increased secondary folding of RNA at low temperatures. In this review established and novel aspects concerning the structure, function and control of these CSPs are discussed. A model for bacterial cold adaptation, with a central role for ribosomal functioning, and possible mechanisms for low-temperature sensing are discussed.

Influence of yeast immobilization on fermentation and aldehyde reduction during the production of alcohol-free beer

Production of alcohol-free beer by limited fermentation is optimally performed in a packed-bed reactor. This highly controllable system combines short contact times between yeast and wort with the reduction of off-flavors to concentrations below threshold values. In the present study, the influence of immobilization of yeast to DEAE-cellulose on sugar fermentation and aldehyde reduction was monitored. Immobilized cells showed higher activities of hexokinase and pyruvate decarboxylase compared to cells grown in batch culture. In addition, a higher glucose flux was observed, with enhanced excretion of main fermentation products, indicating a reduction in the flux of sugar used for biomass production. ADH activity was higher in immobilized cells compared to that in suspended cells. However, during prolonged production a decrease was observed in NAD-specific ADH activity, whereas NADP-specific activity increased in the immobilized cells. The shifts in enzyme activities and glucose flux correlate with a higher in vivo reduction capacity of the immobilized cells.

Rapid fluorescence assessment of intracellular pH as a viability indicator of Clavibacter michiganensis subsp. michiganensis

The viability of Clavibacter michiganensis subsp. michiganensis (Cmm) was determined by measuring the intracellular pH (pHin) as a viability parameter. This was based on the observation that growth of Cmm was inhibited at pH 5.5 and below. Therefore, viable cells should maintain their pHin above this pH value. The pHin of Cmm was determined using the fluorescent probe 5(and 6-)-carboxyfluorescein succinimidyl ester (cFSE). The pHin of Cmm cells exposed to acid treatments was determined using fluorescence spectrofluorometry, and for cells exposed to elevated temperatures, the pHin was determined using fluorescence spectrofluorometry and flow cytometry (FCM). A good correlation was found between the presence of a pH gradient and the number of colony-forming units (cfu) observed in plate counts. However, with the spectrofluorometry technique, the analysis is based on the whole cell population and the detection sensitivity of this technique is rather low, i.e., cell numbers of at least 107 cfu ml-1 are needed for the analysis. Using FCM, heat-treated and non-treated Cmm cells could be distinguished based on the absence and presence of a pH gradient, respectively. The major advantage of FCM is its high sensitivity, allowing analysis of microbial populations even at low numbers, i.e., 102-103 cfu ml-1.

Fluorescence assessment of Lactococcus lactis viability

The reproduction and activity of lactic acid bacteria (LAB) are essential in their applications in the dairy industry and other fermentations. Traditionally used methods like plate counting and acidification tests require long incubation times and provide limited information. Fluorescence techniques provide possibilities for rapid assessment of cell physiology. We used traditional and fluorescence assays to assess the physiological condition of L. lactis subsp. lactis ML3 cultures that were exposed to various stress conditions. After exposure to some of the stress conditions, carboxyfluorescein (cF) labelling did not agree with plate counts. Therefore, a two-step method was developed in which cF labelling was followed by a lactose-energized efflux assay. The combined assay proved to be a good and rapid indicator for reproduction and acidification capacity of stressed L. lactis. This novel assay has potential for physiological research and dairy applications related to LAB.

Assessment of viability of microorganisms employing fluorescence techniques

Viability assessment of microorganisms is relevant for a wide variety of applications in industry, including evaluation of inactivation treatments and quality assessment of starter cultures for beer, wine, and yoghurt production. Usually, the ability of microbial cells to reproduce is considered as the benchmark method for determination of cell viability, and this is most commonly determined by the plate count method. The time needed to form visible colonies, however, is relatively long. Therefore, there is an increasing interest in rapid methods which exploit criteria other than reproduction. In this review the applications of fluorescent probes for, e.g., determination of membrane integrity, enzyme activities, respiration, membrane potential and intracellular pH, are discussed in detail.

Characterization of a novel beta-galactosidase from Bifidobacterium adolescentis DSM 20083 active towards transgalactooligosaccharides

This paper reports on the effects of both reducing and nonreducing transgalactooligosaccharides (TOS) comprising 2 to 8 residues on the growth of Bifidobacterium adolescentis DSM 20083 and on the production of a novel beta-galactosidase (beta-Gal II). In cells grown on TOS, in addition to the lactose-degrading beta-Gal (beta-Gal I), another beta-Gal (beta-Gal II) was detected and it showed activity towards TOS but not towards lactose. beta-Gal II activity was at least 20-fold higher when cells were grown on TOS than when cells were grown on galactose, glucose, and lactose. Subsequently, the enzyme was purified from the cell extract of TOS-grown B. adolescentis by anion-exchange chromatography, adsorption chromatography, and size-exclusion chromatography. Beta-Gal II has apparent molecular masses of 350 and 89 kDa as judged by size-exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively, indicating that the enzyme is active in vivo as a tetramer. Beta-Gal II had an optimal activity at pH 6 and was not active below pH 5. Its optimum temperature was 35 degrees C. The enzyme showed highest V(max) values towards galactooligosaccharides with a low degree of polymerization. This result is in agreement with the observation that during fermentation of TOS, the di- and trisaccharides were fermented first. Beta-Gal II was active towards beta-galactosyl residues that were 1-->4, 1-->6, 1-->3, and 1 <--> 1 linked, signifying its role in the metabolism of galactooligosaccharides by B. adolescentis.

Assessment of the intracellular pH of immobilized and continuously perfused yeast cells employing fluorescence ratio imaging analysis

The intracellular pH (pHin) of Saccharomyces cerevisiae was measured employing fluorescence ratio imaging microscopy (FRIM). The yeast cells were fluorescently labeled with the pH dependent probe 5(and-6)-carboxyfluorescein (cF) or 5(and-6)-carboxyfluorescein succinimidyl ester (cFSE), and subsequently attached to ferric nitrate pretreated glass slides. The labeled and adhered cells could still divide and were metabolically active. Measurement of the pHin was performed during continuous perfusion of the cells with buffer or medium. Cells labeled with cF are highly fluorescent and in non-energized cells the pHin could be easily measured. However, in energized yeast cells cF was accumulated in the vacuoles and/or exported to the extracellular environment, most likely by an energy-dependent transport system, thus limiting the time period over which the pHin can be effectively measured. Therefore, cFSE (which conjugates with aliphatic amines in the cytoplasm) was applied to prevent translocation of fluorescent probe to the vacuole and/or extracellular environment. The continuous perfusion in combination with the cFSE labeling of the immobilized cells was successfully applied to determine the effect of low and high pHin and addition of glucose on the pHin of individual yeast cells over a long time period.

Characteristics of the biologically active 35-kDa metalloprotease virulence factor from Listeria monocytogenes

Listeria monocytogenes, a facultative intracellular pathogen, synthesizes an extracellular protease which is responsible for the maturation of phosphatidylcholine phospholipase C (lecithinase), a virulence factor involved in cell-to-cell spread. This work describes the environmental parameters necessary for increased production of mature, 35-kDa active protease in strains of L. monocytogenes, and its detection using polyclonal antibodies raised against Bacillus subtilis neutral protease. High performance liquid affinity chromatography was exploited to isolate the biologically active form of the mature protease, which was then subjected to biochemical characterization using casein as a substrate. The protease is a zinc-dependent metalloprotease which degrades casein over a wide range of temperatures and pH values. It can also degrade actin, the most abundant protein in many eukaryotic cells. The Listeria protease was shown to exhibit a high thermal stability and a relatively narrow substrate specificity. A three-dimensional model built on the basis of the homology with thermolysin was used to understand the structural basis of these characteristics.

Cold shock proteins and low-temperature response of Streptococcus thermophilus CNRZ302

Low-temperature adaptation and cryoprotection were studied in the thermophilic lactic acid bacterium Streptococcus thermophilus CNRZ302. S. thermophilus actively adapts to freezing during a pretreatment at 20 degrees C, resulting in an approximately 1, 000-fold increased survival after four freeze-thaw cycles compared to mid-exponential-phase cells grown at an optimal temperature of 42 degrees C. No adaptation is observed when cells are exposed to a temperature (10 degrees C) below the minimal growth temperature of the strain (just below 15 degrees C). By two-dimensional gel electrophoresis several 7-kDa cold-induced proteins were identified, which are the major induced proteins after a shift to 20 degrees C. These cold shock proteins were maximally expressed at 20 degrees C, while the induction level was low after cold shock to 10 degrees C. To confirm the presence of csp genes in S. thermophilus, a PCR strategy was used which yielded products of different sizes. Sequence analysis revealed csp-like sequences that were up to 95% identical to those of csp genes of S. thermophilus ST1-1, Streptococcus dysgalactiae, Streptococcus pyogenes, and Lactococcus lactis. Northern blot analysis revealed a seven- to ninefold induction of csp mRNA after a temperature shift to 20 degrees C, showing that this thermophilic bacterium indeed contains at least one cold-inducible csp gene and that its regulation takes place at the transcriptional level.

Microbial stress response in minimal processing

"Bacteria have evolved adaptive networks to face the challenges of changing environments and to survive under conditions of stress. Therefore, the efficiencies of inactivation and preservation methods need to be assessed, especially with regard to the enormous potential of food pathogens to adapt to a wide variety of stress conditions. All adaptive responses, whether to changing nutrients or to various stresses encountered in minimal processing, involve a series of genetic switches that control the metabolic changes taking place. A common regulatory mechanism involves the modification of sigma (sigma) factors whose primary role is to bind to core RNA polymerase conferring promoter specificity directing expression of specialty regulons involved in heat-shock response, the chemotactic response, sporulation, and general stress response. Examples of the latter regulon in Gram-positive bacteria (the sigmaB regulon) and in Gram-negative bacteria (the RpoS regulon) will be discussed in more detail. Cellular adaptive mechanisms to starvation, cold shock, heat shock, (weak) acids, high osmolarity and high hydrostatic pressure will be described and their significance in food preservation and safety will be discussed."

Rapid fluorescence assessment of the viability of stressed Lactococcus lactis

The aim of this study was to establish the use of the fluorescent probes carboxyfluorescein (cF) and propidium iodide (PI) for rapid assessment of viability, using Lactococcus lactis subsp. lactis ML3 exposed to different stress treatments. The cF labeling indicated the reproductive capacity of mixtures of nontreated cells and cells killed at 70 degrees C very well. However, after treatment up to 60 degrees C the fraction of cF-labeled cells remained high, whereas the survival decreased for cells treated at above 50 degrees C and was completely lost for those treated at 60 degrees C. In an extended series of experiments, cell suspensions were exposed to heating, freezing, low pH, or bile salts, after which the colony counts, acidification capacity, glycolytic activity, PI exclusion, cF labeling, and cF efflux were measured and compared. The acidification capacity corresponded with the number of CFU. The glycolytic activity, which is an indicator of vitality, was more sensitive to the stress conditions than the reproduction, acidification, and fluorescence parameters. The cF labeling depended on membrane integrity, as was confirmed by PI exclusion. The fraction of cF-labeled cells was not a general indicator of reproduction or acidification, nor was PI exclusion or cF labeling capacity (the internal cF concentration). When the cells were labeled by cF, a subsequent lactose-energized efflux assay was needed for decisive viability assessment. This novel assay proved to be a good and rapid indicator of the reproduction and acidification capacities of stressed L. lactis and has potential for physiological research and dairy applications related to lactic acid bacteria.